1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus which purifies a NOx in an exhaust gas of an internal combustion engine which is capable of performing combustion at a lean air-fuel ratio.
2. Description of the Related Art
A selective reduction type NOx catalyst which reduces or decomposes the NOx in the presence of hydrocarbon in an ambient atmosphere with excessive oxygen is widely used in the exhaust gas purifying apparatus which purifies the NOx in the exhaust gas of the internal combustion engine which is capable of performing combustion a lean air-fuel ratio (for example, a diesel engine or a lean burn gasoline engine).
A reductant (for example, hydrocarbon (HC)) is required in purification of NOx by the selective reduction type NOx catalyst and therefore, the exhaust gas purifying apparatus of this kind comprises means for supplying a reductant to the selective reduction type NOx catalyst.
An exhaust gas purifying apparatus has been disclosed in Japanese Patent Application Laid-Open No. 6-185342, in which apparatus a HC absorbent which absorbs and desorbs HC is located at an upstream position of the selective reduction type NOx catalyst which is disposed in an exhaust gas passageway in the internal combustion engine, the HC is absorbed by the HC absorbent when a temperature of the exhaust gas is low, while the HC is desorbed from the HC absorbent when the temperature of the exhaust gas is high and the desorbed HC is supplied to the selective reduction type NOx catalyst, so that the NOx is purified.
In the mean time, the selective reduction type NOx catalyst has a purification temperature window which is an optimal range for NOx purification, and when the catalyst temperature is outside this purification temperature window, the NOx purification efficiency is greatly reduced or the catalyst exerts no purification ability. The purification temperature window is as narrow as, for example, a range from 200 to 300.beta.C. and, when the selective reduction type NOx catalyst is used as a catalyst in the exhaust gas purifying apparatus for the internal combustion engine of a vehicle, a temperature of the selective reduction type NOx catalyst has a chance to be outside the purification temperature window when the vehicle is in an operating state of deceleration. On the other hand, when the vehicle is in the operating state of acceleration, not only a great quantity of NOx is produced in the exhaust gas of the internal combustion engine but also an exhaust gas temperature is raised high and, then if the high temperature exhaust gas flows into the HC absorbent located upstream, the HC which has been absorbed in the HC absorbent is desorbed from the HC absorbent and flows into the selective reduction type NOx catalyst located downstream.
On many occasions, the operating state of the vehicle frequently changes between acceleration and deceleration alternately when, for example, running through the streets of a city, and in many cases the vehicle is in the decelerator state immediately before the accelerating operation. When the vehicle operating state is of deceleration and thereby the catalyst temperature of the selective reduction type NOx catalyst is outside the purification temperature window, if the operating state is changed from deceleration to acceleration, a high temperature exhaust gas begins to flow into the HC absorbent immediately after the acceleration operation has started. Therefore, the HC which has been absorbed in the HC absorbent is desorbed and flows into the selective reduction type NOx catalyst, but the catalyst temperature of the selective reduction type NOx catalyst has not been raised yet at a time immediately after the acceleration and it is still outside the purification temperature window, so that the NOx is not purified, and the HC which flows into the selective reduction type NOx catalyst is not utilized for purification of NOx, but merely flows through the catalyst without any effective use thereof.
As described above, the NOx is generated in great quantity in the exhaust gas during the acceleration operation, and the NOx purification should be essentially started immediately after acceleration and from this view point, the conventional exhaust gas purifying apparatus for the internal combustion engine has had a room for further improvement.